The present application relates to a displacement detection apparatus configured to detect a displacement (movement) of a scale using interference of light and relates to a polarization beam splitter and a diffraction grating installed in the displacement detection apparatus.
Recently, fixed-point detection apparatuses and displacement detection apparatuses using light-emitting diodes and laser devices have high resolution that enables a measurement of displacement smaller than 1 nm. Among such displacement detection apparatuses, the displacement detection apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2004-144581 includes a movable scale (12) having a first area (12a) for recording position information at predetermined intervals and a second area (12b) for recording position information at predetermined intervals different from that in the first area (12a). A first reading unit (10) reads a signal of diffracted light from the first area (12a) of the scale (12), whereas a second reading unit (11) reads a signal of diffracted light from the second area (12b) (for example, refer to Japanese Unexamined Patent Application Publication No. 2004-144581). By detecting the phases of the readout signals and comparing each two phases with the displacement detection apparatus, an origin signal indicating the origin position used as a reference of the displacement amount. In other words, the above-described fixed-point detection apparatus is capable of detecting an origin position in this way and is used together with the displacement detection apparatus.
In the displacement detection apparatus according to Japanese Unexamined Patent Application Publication No. 2004-144581, a position for reading out position information from the first area (12a) and a position for reading out position information from the second area (12b) are aligned on an inner line. Thus, an accurate origin signal can be generated without being affected by Abbe error.
Since, in general, with such a displacement detection apparatus, the optical system becomes complex, there is a problem in that the size of the apparatus increases. Therefore, there is a need for reduction in size. Moreover, since the optical system is complex, it is important to maintain high assembly accuracy of the optical system. When the assembly accuracy is low, distortion may be generated in the optical system and the electric-signal processing system when the displacement detection apparatus continuously used.